In variable speed drives of the type having drive and driven sheaves interconnected by a belt, the drive sheave has an axially slidable flange movable to various selected positions relative to a fixed flange for varying the output speed of the drive, and the driven sheave has an axially slidable flange movable relative to the fixed flange in response to the increased or decreased force applied by the belt when the slidable flange of the drive sheave moves toward or away from the fixed sheave. While the slidable flange of the drive sheave is controlled either by a manually operated mechanism or by a power operated mechanism responsive to predetermined conditions, the slidable flange of the driven sheave is normally controlled either by a spring alone or by a torque cam assembly in which a spring may or may not be utilized. The spring alone merely resists the movement of the slidable flange away from the fixed flange and applies a force to urge the slidable flange toward the fixed flange, whereas the torque cam assembly responds to the load on the output of the drive to vary the force urging the slidable flange toward the movable flange and increases the resistance in response to an increase in the load. The torque cam assembly mechanism is normally preferred to the spring alone for the reason that it is directly responsive to the increase in torque resulting from an increase in the load on the output of the drive. This retains the belt tight and produces the required pressure of the belt on the flanges of the driven sheave for optimum power output for any given operating condition.
In the torque cam assembly type, three cams spaced around the shaft of the driven sheave are normally used in order to balance the forces around the shaft and thereby maintain optimum operation of the sheave for extended periods of time by preventing undue wear between the shaft and the sleeve or collar on which the cams are mounted. The use of multiple cams creates a problem, in that effective contact between the cam followers for the various cams is often not equal, thereby preventing equal load sharing on the cams and creating an imbalance of forces between the shaft and the cams, and a consequent increase in wear between the parts, as well as decreasing the response to changes in operating conditions.